Fluid flow sensors in the form of MEMS devices are configured to measure properties of fluid in contact with the sensors and provide output signals representative of the fluid flow rates. Thermal fluid flow sensors are configured to heat the fluid and measure the resulting thermal properties of the fluids to determine flow rates. Such thermal flow sensors generally include a microsensor die consisting of a substrate and one or more elements disposed on the substrate for heating the fluid and sensing the fluid thermal properties.
A microbridge liquid flow sensor, for example as detailed in U.S. Pat. No. 4,651,564 to Johnson et al., is an example of such a thermal flow sensor. The microbridge sensor includes a flow sensor chip which has a thin film bridge structure thermally insulated from the chip substrate. A pair of temperature sensing resistive elements are arranged on the upper surface of the bridge either side of a heater element such that, when the bridge is immersed in the liquid stream, the flow of the liquid cools the temperature sensing element on the upstream side and promotes heat conduction from the heater element to thereby heat the temperature sensing element on the downstream side. The temperature differential between the upstream and downstream sensing elements, which increases with increasing flow speed, is converted into an output voltage by incorporating the sensing elements in a Wheatstone bridge circuit such that the flow speed of the gas or liquid can be detected by correlating the output voltage with the flow speed. When there is no liquid flow, there is no temperature differential because the upstream and downstream sensing elements are at similar temperatures.
However, a major drawback of microbridge liquid sensors is that the performance of the sensors is limited when operating in harsh environments or in high mass or high flow rate conditions. In such conditions, particulates of the flowing gas or liquid being measured are easily deposited in the underlying structure of the microbridge causing uncontrolled thermal changes and even damage to the sensor so that the reliability and operating life time of the sensors can be adversely affected.
Another example of a thermal sensor is a microstructure thermal flow sensor having a microsensor die with a Microbrick® or microfill structure which sensor is more suited to measuring fluid flow and properties under harsh environmental conditions. Note Microbrick® is a registered trade mark of Honeywell Inc. of Morristown, N.J. The microstructure flow sensor uses a Microbrick® which consists of a solid quartz substrate beneath the heating/sensing elements and has a passivation layer isolating the heating/sensing elements from the fluid so that the sensor is less susceptible to the effects of the fluid. Although this type of microstructure sensor is capable of reliable and rapid-response operation under harsh environments making it a versatile fluid flow sensor for a variety of applications, this type of sensor is expensive to make and integrate into existing sensor applications.
The aforementioned problems demonstrate that there is a need to provide a robust low cost fluid flow sensor which can be easily integrated into existing sensor applications.